Once a user terminal has synchronized to a radio cell in a radio communication network, acquired the physical-layer identity of the cell and detected the cell frame timing, the terminal also has to acquire the cell system information. This system information is normally broadcasted repeatedly by the network, and needs to be acquired by user terminals in order for them to be able to access and operate properly within the network and within a specific cell.
Examples of relevant system information include information about downlink and uplink cell bandwidths and/or configurations, parameters related to random access transmission, uplink power control and so forth.
For a better understanding, it may be useful with a brief introduction and overview of an exemplary radio communication system based on LTE (Long Term Evolution).
LTE is a novel radio access technology being standardized by 3GPP. Only the Packet Switched (PS) domain will be supported by LTE, i.e. all services are to be supported in the PS domain. The standard will be based on OFDM (Orthogonal Frequency Division Multiplexing) in the downlink and SC-FDMA (Single Carrier Frequency Domain Multiple Access) in the uplink.
The LTE radio access architecture is based around the LTE radio base stations, referred to as eNodeB:s, which communicate with mobile terminals, also referred to as User Equipment (UE). One of the basic principles of LTE radio access is shared-channel transmission in which time-frequency resources are dynamically shared between users.
With reference to FIG. 1, in the time domain, one sub frame of 1 ms duration is divided into 12 or 14 OFDM (or SC-FDMA) symbols, depending on the configuration. One OFDM (or SC-FDMA) symbol includes a number of sub carriers in the frequency domain, depending on the channel bandwidth and configuration. One OFDM (or SC-FDMA) symbol on one sub carrier is referred to as an RE (Resource Element). FIG. 1 is valid for an example with 2 antennas. If by way of example 4 antennas are used, twice as many reference symbols will be transmitted.
In LTE no dedicated data channels are used, instead shared channel resources are used in both downlink and uplink. These shared resources, DL-SCH (Downlink Shared Channel) and UL-SCH (Uplink Shared Channel), are controlled by scheduling functionality that assigns different parts of the downlink and uplink shared channels to different UEs for reception and transmission respectively.
The assignments for the DL-SCH and the UL-SCH are transmitted in a control region covering a few OFDM symbols in the beginning of each downlink sub frame, as indicated in FIG. 1. The DL-SCH is transmitted in a data region covering the rest of the OFDM symbols in each downlink sub frame. The UEs will be required to monitor the control region to be able to detect the assignments directed to them in the data region. The assignments in the control region are typically carried by Physical Downlink Control Channels (PDCCHs). The downlink shared channel available for data transfer in the data region is made up of the Physical Downlink Shared Channel (PDSCH).
In Long Term Evolution (LTE) systems, for example, system information is generally delivered by two different mechanisms relying on two different transport channels:                A limited amount of system information, corresponding to the so-called Master Information Block (MIB), is transmitted using the so-called Broadcast Channel (BCH).        A larger part of system information, corresponding to different so-called System Information Blocks (SIBS), is transmitted using the downlink shared channel (DL-SCH).        
The MIB transmitted using BCH generally includes such system information that is absolutely necessary for a user terminal to be able to read the remaining SIB system information provided using DL-SCH.
FIG. 2 is a schematic diagram illustrating the mapping between logical channels, transport channels and physical channels for the downlink in the particular example of LTE.
The Medium Access Control (MAC) layer offers services to the Radio Link Control (RLC) layer in the form of logical channels. A logical channel is generally defined by the type of information it carries and is normally classified as a control channel, used for transmission of control and configuration information necessary for operating an LTE system, or a data channel used for user data. The set of logical channels defined for LTE includes:                Paging Control Channel (PCCH) used for paging mobile user terminals, also referred to as User Equipment (UE).        Broadcast Control Channel (BCCH) used for transmission of system information from the network to all mobile user terminals in a cell.        Common Control Channel (CCCH) used for transmission of control information in conjunction with random access.        Dedicated Traffic Channel (DTCH) used for transmission of user data to/from a mobile terminal.        Dedicated Control Channel (DCCH) used for transmission of control information for individual configuration of mobile terminals.        Multicast Traffic Channel (MTCH) used for downlink transmission of Multimedia Broadcast and Multicast Services (MBMS).        Multicast Control Channel (MCCH) used for transmission of control information required for reception of MTCH.        
A similar logical channel structure is used for Wideband Code Division Multiple Access (WCDMA) and/or High Speed Packet Access (HSPA) systems. Compared to WCDMA/HSPA, LTE has a somewhat more simplified logical channel structure with a reduced number of logical channel types.
The physical layer offers services to the MAC layer in the form of so-called transport channels. A transport channel is generally defined by how and with what characteristics the information is transmitted over the radio interface. For example, for LTE, the following transport channels are defined for the downlink:                Paging Channel (PCH) is used for transmission of paging information from the PCCH logical channel.        Broadcast Channel (BCH) is used for transmission of parts of the BCCH system information, including the MIB block.        Downlink Shared Channel (DL-SCH) is the main transport channel, as previously mentioned.        Multicast Channel (MCH) is used to support MBMS services.        
Each transport channel is mapped to a corresponding physical channel:                Physical Broadcast Channel (PBCH) carries part of the system information required by the terminals to access the network.        Physical Downlink Shared Channel (PDSCH) is the main physical channel for unicast transmission, and also for paging information.        Physical Multicast Channel (PMCH) is used for Multicast/Broadcast over Single Frequency Network (MBSFN) operation.        
It should though be noted that there are also physical channels without a corresponding transport channel, especially for Downlink Control Information (DCI):                Physical Downlink Control Channel (PDCCH) is used for various downlink control information.        Physical Hybrid-ARQ Indicator Channel (PHICH) carries the hybrid-ARQ acknowledgment to indicate whether or not a transport block should be retransmitted.        Physical Control Format Indicator Channel (PCFICH) provides information necessary to decode the PDCCH.        
There is a corresponding mapping (not shown) of logical channels, transport channels and physical channels for the uplink as well.
In LTE, for example, user equipment (UE) can be in two different states on Radio Resource Control (RRC) level, as illustrated in FIG. 3.
RRC_CONNECTED is the state used when the UE is active and connected to a specific cell within the network. RRC_CONNECTED can be said to have two sub-states, IN_SYNC and OUT_OF_SYNC, depending on whether or not the uplink is synchronized to the network.
RRC_IDLE is a so-called low activity state in which the UE sleeps most of the time in order to reduce battery consumption. Uplink synchronization is not maintained, and the only uplink transmission activity that may take place is so-called random access to move from RRC_IDLE to RRC_CONNECTED. In the downlink, the UE can wake up periodically to monitor the Paging Channel (PCH) according to a Discontinuous Reception (DRX) cycle in order to be paged for incoming calls, as will be explained in more detail below.
In addition to assignments for DL-SCH and UL-SCH, also assignments for the Paging Channel (PCH) are carried by PDCCHs in the control region. The PCH is used to transmit paging information to UEs in RRC_IDLE and/or to inform UEs in RRC_IDLE and UEs in RRC_CONNECTED about a system information change. A UE may verify that acquired system information remains valid by either regularly checking a certain value or, by looking for the mentioned system information change indication in the paging messages.
As mentioned above, LTE allows Discontinuous Reception (DRX) for the UEs in order to save UE battery. FIG. 4 is a schematic diagram illustrating the basic principles of the DRX mechanism. The DRX mechanism is used for allowing the UE terminal to sleep most of the time, with the UE receiver circuitry switched off, and only periodically wake up for a brief period of time to monitor the paging channel. As illustrated in FIG. 4, a DRX cycle period normally includes a short so-called On Duration period followed by a relatively longer Sleep period. For UEs in RRC_IDLE a DRX pattern aligned to the basic paging schedule is applied on a group basis for a set of UEs. The DRX pattern is aligned to the paging schedule in such a way that the UE has a possibility to read the paging messages while awake rather than while in the battery saving DRX sleep mode.
In order to further reduce the battery consumption of the UEs, DRX functionality for UEs in RRC_CONNECTED may also be applied. Several parameters, resulting in a huge amount of different possible configurations, have been standardized for this purpose.
Applying a DRX pattern for RRC_CONNECTED UEs aligned to the DRX pattern for RRC_IDLE UEs makes it possible for a UE to detect the paging messages, including system information notifications, in both RRC_CONNECTED and RRC_IDLE. Unfortunately, it turns out that this approach leads to problems with respect to network performance and operation.
Similar problems related to distribution of system information notifications and/or indications can also be found in other radio communication networks having user equipment operating based on Discontinuous Reception (DRX) in connected and idle modes.